1. Field of the Invention
This invention relates to a heavy duty pneumatic radial tire, and more particularly to a heavy duty pneumatic radial tire for use in truck and bus in which cracks liable to be created at an end portion of a cord layer in a belt and growth thereof are controlled at and minimized to favorably suppress the occurrence of separation failure due to the crack growth when the tire is applied to a heavy duty vehicle such as truck and bus.
2. Description of Related Art
It is well-known that as the running distance of the pneumatic radial tire mounted onto the heavy duty vehicle such as small-large size truck and bus increases, cracks are created at a cord end of a narrow-width cord layer in cross cord layers having different widths, cords of which layers being crossed with each other at a relatively small cord angle with respect to an equatorial plane of the tire, among plural cord layers constituting the belt. The crack initially created is minute like "pecking" of the cord end, but such a minute crack propagates to a big crack along the cord as the running distance of the tire increases. When the crack growth progresses in a certain extent, the front end of the crack grows toward a cord end of the adjoining cord and finally cracks are connected to each other over substantially a full periphery of the cord layer along a side edge of the cord layer. If the crack growth is advanced to this stage, a running time is not taken so much until the separation failure is created between the cross cord layers.
Accordingly, a period ranging from the occurrence of minute crack at the cord end to the growth of connected crack along the side edge of the cord layer on its periphery controls a service life of the tire through belt separation failure. For this end, there are proposed various improving structures of the tire directed to the prolongation of tire running time (running distance) though the connection of cracks is unavoidable.
As the improving structure, JP-A-4-183605 proposes a heavy duty pneumatic radial tire wherein a cushion rubber is arranged between end portions of second and third cross steel cord layers counted from a side of a carcass in a belt comprised of 3 to 4 steel cord layers and an upper side of an end portion of the third layer is covered with a rubber layer having a thickness of not less than 1.5 mm to overlap over a distance of not less than 20 mm from the end of the layer and the rubber layer is made from rubber having a JIS hardness of 65-75 and a modulus at 300% elongation of 130-200 kgf/cm.sup.2.
Furthermore, JP-A-4-252705 discloses a heavy duty pneumatic radial tire wherein a belt edge cushion rubber is interposed between end portions of two cross steel cord layers in a belt comprised of four steel cord layers and such end portion is covered with a rubber sheet having a thickness of 1.0-3.0 mm and 50% modulus of the rubber sheet is smaller than that of a coating rubber for the steel cord layer but larger that that of a base tread rubber. Moreover, JP-A-6-320906 proposes a heavy duty pneumatic radial tire wherein a belt comprised of four steel cord layers is arranged between a base tread rubber of low modulus and a carcass and a cushion rubber is disposed between end portions of second and third cross cord layers counted from a side of the carcass and the end portion is covered with a rubber sheet having a thickness of 0.8-3.3 mm likewise the case of JP-A-4-252705 and further the rubber sheet has a JIS hardness lower than those of rubber for the second and third cord layers and 50% modulus larger than that of the base tread rubber.
However, the effect of sufficiently controlling the crack growth under severe service conditions of heavy loading, running speed and the like can not be attained even by any improving structure of the belt described in these prior proposals, so that the belt separation failure is naturally liable to be caused even in these new tires. Therefore, even if the tire is recapped, it is obliged to prematurely create such a failure. Although this type of the heavy duty pneumatic radial tire particularly tends to be important in the recapping adaptability, even when the above new tire is completely run, the occurrence of the big crack unsuitable for the recapping is frequently observed at the end portion of the belt. Therefore, it is demanded to further improve the crack resistance and resistance to crack growth at the end portion of the belt and hence considerably improve the resistance to belt separation.